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BJT base current 1/f noise

Is horrible.

I'm just debugging a nice diode laser controller for one customer and getting ready to do another one for another (much better) customer. It uses a ZXTP25020 PNP with a biggish degeneration resistor and two-pole bypassing of the base to get low noise at high frequency.

Turns out the 1/f noise is atrocious. The total noise from 2 Hz on up is only a couple of PPM, but it should be a factor of 10 better than that. With a total base resistance of 1k, the 1/f corner at 40 mA I_C is over 10 kHz.

The bias network looks like this:

+9V 0----------*--------*-------* | | | | | | | | R | 4.7 | 3.3 R 39 ohm TF | nF | nF R CCC CCC | CCC CCC | | | | / | | |< ZXTP25020 From 0--RRRR--*--RRRR--*---- | Op Amp |\ 499 499 | \ | | --- \ / --> V --> ----- | | GND

The emitter resistor can drop up to about 5V.

I've got some higher-beta transistors on order (FZT788B), and will reduce the impedance of the bias network, but I was fairly shocked that the 1/f noise was so bad.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
https://hobbs-eo.com
Reply to
Phil Hobbs
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What! Can't toyLTspice spit out the noise values for each of the components _including_ the PNP parameters ?>:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
     It's what you learn, after you know it all, that counts.
Reply to
Jim Thompson

That's a pretty fast transistor; could it be oscillating?

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

The simulator can only use what the models provide. Or did you think that pspice was from Hogwarts?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Don't think so. I have a 1 GHz scope across the laser, and didn't see anything like that, and have applied thermal-digital analysis(*) to the whole thing. I'll try going to 49.9 ohms and 47 nF and see if it gets better, which it should. I might need to use a TCA0372 booster to drive

5-ohm resistors. :(

Cheers

Phil Hobbs

(*) i.e. heat gun, cold spray, and fingers

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

I know. I was just yanking your chain ;-)

It seems, anymore, that even discrete device models are seriously lacking when it comes to noise.

And MOS noise models for chip devices are 95% "default" :-( ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
     It's what you learn, after you know it all, that counts.
Reply to
Jim Thompson

Yeah, the base current noise into the 1K resistor string could make a lot of noise... assuming the signal from the opamp is perfect.

Is the overall feedback optical?

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Are you sure your 9V supply is quiet, or that the op amp is quiet relative to that supply? Even if you already have one of your capacitance multipliers on the supply, lowering the filter cutoff frequency might give a quick verification that it really is 1/f.

Thanks for the on-topic question! Frank

Reply to
Frank Miles

Why?

I had a deko at the data sheet from Diodes Inc. It don't have any spec for

1/f. So, again, what should one expect it to be....?

-- Kevin Aylward

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- SuperSpice
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Reply to
Kevin Aylward

Eventually, but at the moment it's coming from the emitter resistor, so that the loop doesn't null out the base current noise even within its bandwidth.

The idea is to make the bias current way sub-Poissonian, which if done right results in useful amounts of amplitude squeezing of the light, i.e. your detected photocurrent can actually be a few dB quieter than full shot noise if you keep the optical loss low enough. It's more of a checklist feature for most uses, of course, but as you've pointed out there are a lot of crappy diode laser controllers out there, selling for cheap, so building one that just makes the problems magically go away is attractive.

Building a new-to-me kind of high dynamic range system always seems to produce a bit of buried treasure at first. The ZXTP25020 has super-low Rbb' and Ree', so it would be an excellent candidate except for the 1/f noise. If the higher-beta device and lower drive impedance doesn't improve it enough, I'll have to try Darlingtonizing it with one of those nice SiGe:C devices to return the base current to the collector circuit.

That'd need a board spin though, and I'm super busy at the moment.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

+9V 0----------*--------*-------* | | | | | | | | R | 4.7 | 3.3 R 39 ohm TF | nF | nF R CCC CCC | CCC CCC | | | | / | | |< ZXTP25020 From 0--RRRR--*--RRRR--*---- | Op Amp |\ 499 499 | \ | | --- \ / --> V --> ----- | | GND

Both are a lot quieter than the collector current!

The acid test was putting a 1-uF film cap across the 3.3 nF to make the output stage bandwidth drop to about 160 Hz--the 1/f noise didn't change at all. That's pretty diagnostic for a base current problem, as you say.

Well, some of us still work for a living. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Would a hypothetical P-channel BF862 (rare as hens' teeth, I know) do as well?

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams

They never have decent specs for 1/f, and their models never seem to include it either, so you just have to suck it and see. The FTZ788B has the highest guaranteed beta of any suitable transistor I could find, so it seemed like a good thing to try. The SiGe:C Darlington thing will be next on the list if reducing the base-circuit impedance doesn't solve it. Nice discretes seem to be disappearing fast though.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

The late lamented BF862 had a 1/f corner of about a kilohertz, but its voltage noise never got below about 0.8 nV in 1 Hz. A BJT with super low Rbb' and Ree' and running at highish I_C can do a lot better than that.

Within the loop bandwidth, I could suppress the noise by putting the sense resistor in the collector circuit, but then I'd have full shot noise at high frequency, which is what I'm trying to avoid. I'm dropping as much as 5V across the emitter resistor to try to get way below the shot noise.

There are more complicated things I can do, but that takes actual work, and I'm working Saturdays as it is.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

snip

What is the resistor in your bias circuit made of? (type)

Reply to
Long Hair

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Reply to
Lasse Langwadt Christensen

They're all thin (metal) film, precisely to reduce 1/f noise. Good thought though.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

The resistors are all thin film. Thick film ones are different-looking, for one thing.

Maerki's data are interesting though, thanks. I expect that it would be interesting to measure the 1/f noise and drift of just the batteries, using the cross-power-spectrum method. Down in the

Reply to
Phil Hobbs

I wonder what the noise is on these...

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Reply to
Long Hair

Am 11.02.2018 um 15:25 schrieb Phil Hobbs:

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My 89441A that I use as a FFT analyzer is severely 1/f challenged. I'm looking for something better.

I'm working on a chopper amplifier that seems to break the 100pV/rt HZ, but my FFT analyzer hides just that where it starts to be interesting.

But it could do cross spectrum. Since the 1/f seems to come from the sample clock and not from the inputs, there still is hope.

cheers, Gerhard

Reply to
Gerhard Hoffmann

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